[Stoneage_Caddy]

Knocks in the 4.9 are normally becuse of the undersize stock #1 main bearing.

2 things you can do

1 switch over to mobil 1 DELVAC 15w40 diesel truck oil. This works pretty well but wont get rid of the knock completly, normally this is recomended for the 4100 engines becuse of the distributor gear. But it works well for all the pushrod 4.x liter engines. Mine had nothing but devlac 15w40 since i got it in 2003 till the last oil change. No job so i put in chevron , i regret it but moneys tight. now i have oil leaks again....

the other thing you can do once your 100% sure it is the main bearing is to replace them. took me a week on the garage floor to change out all 5 , 4 of them replaced with stock and the number 1 replaced with the upsized GM service unit.

I would go with the oil first , esp if the lucas helped. Be sure to use a GM filter also.